Field of the Invention
The invention relates to a projection device.
Description of Related Art
A projection device that forms an image on a projection surface by light projection utilizes multiple types of semiconductor laser elements, such as red, green, and blue, to project a light of the desired color. However, even with application of a driving current to the semiconductor laser elements, a certain time is required before a carrier having the concentration that enables laser oscillation can be generated. For this reason, a light emission delay may occur and it may take a rise time for the outputted light to reach a light amount corresponding to the driving current. The light emission delay refers to a phenomenon that, at the beginning of application of the driving current, only a low amount of light is outputted from the semiconductor laser elements and it takes time for the outputted light to reach a constant light amount that corresponds to the current value of the applied driving current. Particularly, in recent years, a system that uses a red semiconductor laser element of 600 [nm] and a green semiconductor laser element of 500 [nm] has been put into practical use. These semiconductor laser elements facilitate occurrence of a long rise time in comparison with the conventional semiconductor laser elements of the 1.3 μm band, 1.5 μm band, or 780 nm band. The reason is that these semiconductor laser elements have the characteristics of requiring more time to generate the carrier having concentration that enables laser oscillation.
Thus, in Patent Literature 1 (Japanese Patent Publication No. 2012-209380), for example, an auxiliary current is added to the driving current at the start of light emission to compensate for the light emission delay of the semiconductor laser However, for the light projection device that includes multiple semiconductor laser elements, how much rise time is required may differ depending on the types of the semiconductor laser elements. For this reason, when the semiconductor laser elements have different degrees of light emission delay, the timings at which the semiconductor laser elements output the light having the light amount corresponding to the driving current deviate from one another and color unevenness may occur in the edge portion of the image displayed on the projection surface. For instance, among the multiple types of semiconductor laser elements, the light emission delay tends to occur on red and green semiconductor laser elements. If the semiconductor laser elements emit light at the same time to project a pure white image, since the blue semiconductor laser element does not have any light emission delay, the projected light has color unevenness. In particular, the edge portion of the image turns bluish.
Regarding such issues, Patent Literature 1 has not mentioned making compensation for the different degrees of light emission delay of multiple types of semiconductor laser elements. In addition, the light amount may easily overshoot and it is difficult to adjust the light amount at the start of light emission. Besides, power consumption will increase corresponding to application of the auxiliary current, which is also a problem.